The interconversions of the various forms of the guinea pig and rabbit progestin receptor system may best be described as: 4.5S reversibly yields 5.5S reversibly yields 7S reversibly yields approximately 10S Our sedimentation data indicate that one function of a progestin (in additon to stabilizing the 7S form) is to drive the equilibria to the right. These researches are designed to substantiate this universal model by determining the association and dissociation rate constants for each interconversion, thereby we shall determine one effect of the steroid on the receptor. Taking advantage of the stabilizing properties of the progestin we shall also isolate and characterize each of the forms of the receptor. This will be accomplished by hydroxylapatite chromatography, G-100 chromatography, reparative sucrose gradient centrifugations and two dimensional electrophoresis.